This invention generally pertains to processes for injection molding. More specifically, the present invention relates to a method for a fluid assisted injection molding of plastic materials.
The invention is particularly applicable to a gas assisted injection molding process in which a nozzle is utilized to inject a viscous fluid, such as a molten plastic, into a mold cavity together with a non-viscous fluid, such as a gas. However, it will be appreciated by those skilled in the art that the invention has broader applications and may also be adapted for use in many other injection molding environments where both a relatively viscous fluid, such as a plastic or wax, and a relatively non-viscous fluid such as a gas or liquid, are injected into a mold cavity.
Recently, gas assisted injection molding has gained popularity. In this process, the mold cavity is filled with a plasticized thermoplastic material, generally to a volume less than 100% of the mold cavity, and an inert gas is injected under pressure into the plasticized material to fill the rest of the volume in the mold cavity. The gas is injected into the center of the flow of plastic but does not mix with the melt and instead runs along specially designed channels. In this way, with a suitably designed part, a continuous network of hollowed out sections can be provided. The material displaced by the gas from the middle of the sections moves out to fill the remainder of the mold space.
This network of gas channels provides a uniform pressure distribution system throughout the mold space during part rehardening and cool down, thus minimizing internal stresses within the part. Gas injection provides a solution to a number of problems that have long plagued the injection molding industry. These include reducing stress and warpage of the plastic part, eliminating sink marks and providing smooth surfaces on the injection molded part. In addition, clamp tonnage requirements of the mold halves can be reduced in comparison to conventional injection molding processes. The gas injection molding process also permits differing wall thicknesses on a part and faster cycle times in comparison with the conventional injection molding processes. Also, gas assisted injection molding reduces the need for external flow runners.
Several types of nozzles are known for gas assisted injection molding. However, many of these nozzles do not vent the gas back through the nozzle when the discharge of the gas is required. Even those nozzles which do vent the gas back through the nozzle are unsatisfactory because the molten plastic remaining in the nozzle, or in the sprue and runner system, is frequently vented back along with the gas thus causing one of the major difficulties with gas assisted injection molding, namely the plugging of gas channels in the nozzle with thermoplastic material which solidifies and blocks off any further gas flow through these channels. In addition, the gas piping and valves downstream from the nozzle can become plugged. The nozzle then becomes unuseable until it is cleaned out, which is a time-consuming, difficult and expensive process.
Current gas assisted injection molding processes do not allow an injection molding nozzle to operate day in and day out without plugging. Therefore, the nozzle frequently needs to be cleaned as explained above. Conventional gas assisted injection molding processes also do not allow a cleaning of the nozzle without disassembly thereof. In addition, the current gas assisted injection molding processes do not allow a recycling of the gas while preventing various molding chemicals that are used in the molding process from being recycled along with the gas.
Accordingly, it has been considered desirable to develop a new and improved gas assisted injection molding process which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.